Method and apparatus for electrostatic printing using triboelectric inking developers

ABSTRACT

An electrostatic coating and printing process and apparatus wherein electrically conductive carrier particles and electrically non-conductive toner particles are actuated by an electric field to cause at least some of the carrier particles and toner particles to impinge upon a substrate for applying a selective coating thereto. A stencil screen may be interposed along the path of movement of the particles such that the substrate is coated in a pattern defined by apertures in the stencil screen.

United States Patent 1191 1111 3,828,670 Kennedy 1*Aug. 13, 1974 [54] METHOD AND APPARATUS FOR 2,874,063 2/1959 Greig 101/D1G. 13 ELECTROSTATIC PRINTING USING 2,930,351 3/1960 Giaimi l0l/D1G. 13 2,940,864 6/1960 Watson I01/D1G. 13 TRIBOELEURIC INKING DEVELOPERS 2,965,573 12/1960 Gundlach 101/D1G. 13 [75] Inventor: John B. Kennedy, Oak Forest, 111. 3,013,703 12/1961 Hunt 101/DlG, 13 3,081,698 3/1963 Childress et a1. 10l/D1G. 13 1 1 Asslgneel Contlnemal Can Company, New 3,124,457 3/1964 Schwertz 101/1310. 13 York, NY 3,321,768 5/1967 Byrd 101/1310. 13 1 Notice: The portion of the term of this 3,504,624 4/1970 Kennedy 101/129 X patent subsequent to Apr. 7, 1987, has been disclaimed. Primary Examiner-Robert E. Pulfrey Assistant ExaminerE. l-l. Eickholt [22] 1970 Attorney, Agent, or Firm-Diller, Brown, Ramik & [21] Appl. No.: 11,952 Wight Related US. Application Data [60] Division of Ser. No. 772.438, Oct. 31, 1968, Pat. No. 57 ABSTRACT 3,504,624, which is a continuation of Ser. No. 386,182, July 30, 1964, abandoned. An electrostatic coating and printing process and apparatus wherein electrically conductive carrier parti- [52] US. Cl l01/ll4, 101/l I/DIG- l cles and electrically non-conductive toner particles 1 18/637 are actuated by an electric field to cause at least some [51] Int. Cl B41f 15/00, B05b 5/02 of the carrier particles and toner particles to impinge [58] Field of Search 101/1 14, 129, DIG. 13, upon a substrate for applying a selective coating 101/426; 178/66 A; 1 18/637; 340/74 ES; thereto. A stencil screen may be interposed along the 117/37 LE path of movement of the particles such that the substrate is coated in a pattern defined by apertures in the [56] References Cited stencil screen.

UNITED STATES PATENTS 9 Cl 0 7 D F 2,787,556 4 1957 Haas 10l/DlG. 13 gums o--.///////////////ZZ] METHOD AND APPARATUS FOR ELECTROSTATIC PRINTING USING TRIBOELECTRIC INKING DEVELOPERS CROSS REFERENCE TO RELATED APPLICATIONS The present case is a division of application Ser. No. 772,438, filed on Oct. 31, 1968, now US. Pat. No. 3,504,624, which in turn is a continuation of Ser. No. 386,182, filed on July 30, l964and now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to deposition of selective coatings or patterns upon substrates by using electric fields to provide the energy needed for moving the particles from a first location into contact with the substrate.

2. Description of the Prior Art It is known to employ an electric field to effect movement of particles for deposit in a pattern upon a substrate, with the pattern being determined'by previously providing the particles in a pattern upon a first surface and then moving the same therefrom to the substrate; to employ a selectively charged substrate so that some parts thereof attract and hold particles while other parts thereof do not; and to bring the particles to a charged pattern screen of conductive material whereby the particles are charged and then are attracted toward a substrate which, if of conductive material, is polarized oppositely to the charged particles and, if nonconductive material, is displayed in front of a backing electrode charged oppositely to the particles.

SUMMARY OF THE INVENTION According to the instant invention, a mixture of coarse electrically conductive carrier particles and relatively fine electrically non-conductive toner particles is prepared, wherein one or more toner particles adhere to each carrier particle to form a developer mixture, the adhesion occurring by triboelectric effects when the particles are mixed. A base electrode is electrically charged so that such developer mixture in contact therewith has the conductive carrier particles charged therefrom, so that a repulsion effect is established to cause the developer mixture to be repelled away from the base electrode. A stencil screen is presented in the path of the developer mixture so as to form a desired pattern. When the substrate is of conductive material, it is charged oppositely to the carrier particles for controlling the paths thereof from the base electrode to the substrate; when the substrate is of nonconductive material, a backing electrode so charged is placed at the rear thereof.

There are distinctive advantages arising from the use of conductive carrier particles and non-conductive toner particle. Non-conductive toner particles can be made from material of low melting temperature. Thus, the materials can generally be fused to printed objects more readily than available conductive toner particles. In addition, the toner particles may acquire considerably greater velocities by the method and apparatus of the present invention than they would if used alone. The large conductive carrier particles provide conductive charging because of contact with the base electrode, and can attain relatively large terminal velocities.

Accelerating the particles prior to introduction to the stencil screen insures that the velocity vector associated with the particles is properly oriented, i.e., perpendicular to the stencil screen at time of passage of the particles therethrough. This insures that minimal dispersion of the pattern will occur during transport. These two characteristics high magnitude and proper orientation of the velocity vector at the time of introduction through the stencil screen are unique with methods and apparatus exemplified by the present invention.

An object of the present invention is to provide an electrostatic process for applying a selective coating to a substrate comprising the steps of mixing electrically conductive carrier particles with electrically nonconductive toner particles so that the toner particles triboelectrically adhere to the carrier .particles, placing the carrier particles and toner particles in a space between the substrate and a base electrode spaced therefrom, and establishing an electric field in the space between the substrate and base electrode for moving the carrier particles and toner particles toward the substrate.

A further object of the present invention is to provide an electrostatic process as defined above including the step of obstructing the motion of some of the carrier particles and toner particles, and causing the remaining carrier particles and toner particles to continue toward and impinge upon the substrate.

Another object of the present invention is to provide an electrostatic process as defined above wherein the step of obstructing the motion of some of the carrier particles and toner particles includes the step of interposing an apertured stencil screen in the space between the base electrode and the substrate.

I Another object of the present invention is to provide apparatus for coating a substrate with a selective coating of particles wherein the apparatus comprises a pair of spaced electrodes, a power source connected across the electrodes for creating an electric field therebetween, a developer mixture comprised of electrically conductive carrier particles and electrically nonconductive toner particles triboelectrically adhered together and located inthe space between the electrodes, and an apertured stencil screen interposed between the developer mixture and one of the electrodes.

A further object of the invention is to provide apparatus as described above for printing upon electrically conductive substrates.

A still further object of the invention is to provide apparatus as described above particularly adapted for coating, or printing, upon substrates which are formed of electrically non-conductive material.

A further object of the invention is to provide apparatus as described above wherein the apertured stencil screen has openings for defining a pattern, the openings being large enough to allow passage of carrier particles and toner particles therethrough.

With the above and other objects in view that will hereinafter appear, the nature of the invention will be more clearly understood by reference to the following detailed description, the appended claimed subject matter and the several views illustrated in the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a diagrammatical view illustrating a single developer particle which is comprised of a large carrier particle and a plurality of small toner particles.

FIG. 2 is a diagrammatical apparatus assembly for a first form of practice according to the invention, wherein the substrate to be printed on is formed of electrically non-conductive material.

FIG. 3 is a diagrammatical showing of apparatus wherein the substrate to be printed on is comprised of electrically conductive material, and the substrate is utilized as one of a pair of electrodes.

FIG. 4 is a diagrammatical view of apparatus, similar to the view of FIG. 2, and illustrates another form of practice wherein a separate small-mesh screen is interposed between the stencil screen and the substrate.

FIG. 5 is another diagrammatical view of an apparatus assembly, similar to the views in FIGS. 2 and 4, illustrating an embodiment of the invention wherein a conductive metal screen stencil embodies the pattern to be printed.

FIG. 6 is a diagrammatic side elevational view of another form of apparatus.

FIG. 7 is a sectional view taken on the line 7-7 of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS An essential element of the present invention is a developer mixture, one particle of the developer mixture being generally indicated by the numeral 10, and being comprised of a large electrically conductive carrier particle 11 having a plurality of small electrically nonconductive toner particles 12 adhered thereto. The carrier particles 11 are formed of such material as charcoal, metal shot made of various materials, such as steel, aluminum, copper, or small granular iron filings of a range from around 25 to 1,000 microns.

The toner particles 12 are of commercially available types manufactured by the Xerox Corporation. These electrically non-conductive toner particles 12 consist essentially of pigmented or dyed particles of a mixture of n-butyl methacrylate (4l percent) and polystyrene (59 percent), with an average particle size of about 17 microns. As is apparent, the carrier particles 11 are larger than the toner particles 12 and a plurality of toner particles may form an interrupted coating or layer upon the carrier particles and adhere thereto because of triboelectric forces.

In the practice according to FIG. 2, a base electrode 15 and a back electrode 16 comprise spaced parallel plates of electrically conductive material. A patterned stencil screen 17, shown as being formed of nonconductive material such as a sheet of methylmethacrylate polymer about one-quarter inch thick for stiffness, is positioned between and spaced from the base electrode 15 and back electrode 16, and has apertures or openings 18 therethrough for establishing the pattern to be printed or coated. A substrate 19 to be printed, or coated, is illustrativelya sheet of paper, or cardboard, and is positioned parallel to the electrodes and stencil screen, and spaced, e.g., one-quarter inch from the stencil screen 17. While the substrate 19 is shown as being spaced from the back electrode 16, it is to be understood that the substrate 19 can be in contact with the back electrode 16.

A layer of developer mixture 10 is carried on the base electrode 15, preferably in a solid layer completely underlying the stencil screen 17, and consists of a plurality of carrier particles 11 and toner particles 12. As previously pointed out, the toner particles 12 adhere to the conductive carrier particles 11 by triboelectric forces.

The base electrode 15 is preferably connected to the negative side of a high voltage, direct current and the back electrode 16 is connected to the positive side of the high voltage power source 20 by a suitable switch 21. The potential drop between the base electrode 15 and the back electrode 16 may be 2,000 volts but is preferably much larger. Various factors must be considered in determining the preferred potential drop, such as the size and mass of the carrier particles 11 and toner particles 12, the distance between the base electrode 15 and the back electrode 16, and the distance between the base electrode 15 and the substrate 19.

The carrier particles 11 in contact with the base electrode 15 are charged, by conduction, so that the developer mixture 10 is repelled from the base electrode 15 and attracted by the oppositely charged back electrode 16, being influenced during their passage by an electric field established between the electrodes. The charged particles move and are accelerated upwardly as is indicated by the arrows shown in FIG. 2. The particles which come to the openings 18 are passed therethrough and come into contact with the substrate 19 such that the particles are deposited thereon in a pattern determined by the openings 18. The particles which encounter the body of the stencil screen 17 are stopped, and prevented from reaching the substrate 19.

In practicing the invention according to the embodiment which is illustrated in FIG. 3, a substrate 19 can be made of electrically conductive material, e.g., a sheet of metal, and be electrically connected to the power source 20. In this embodiment, the back electrode 16 can be dispensed with because the substrate 119 performs the function of the back electrode. Developer mixture which passes through openings 18 in stencil screen 17 are caused to impinge upon the substrate 19' collision with which causes a considerable portion of the toner particles to overcome the triboelectric effect and become deposited upon the substrate 19'. In the absence of any adhesive coating on the substrate 19, the conductive carrier particles are reversed in polarity, by conduction, and are caused to move back toward the stencil screen 17 and base electrode 15. In the practice according to FIG. 4, a conductive metal screen electrode 25 is provided between the base electrode 15 and the substrate 19, either above the stencil screen 17, as illustrated, or may be positioned below the stencil screen 17. Apertures or openings in the metal screen 25 are of smaller size than the carrier particles 11 and are of larger size than the'toner particles 12. Thus, the metal screen 25 will mechanically stop the carrier particles 11 while the momentum of the toner particles 12 causes the toner particles to detach from the carrier particles 11 and pass through the metal screen 25 and be deposited upon the substrate 19.

In the practice according to FIG. 5, the functions of the pattern stencil screen 17 and conductive metal screen 25, of FIG. 4, are combined by employing a conductive screen 30 with parts of its surface covered by a pattern coating 31 with openings 32 therethrough.

In the practice according to FIGS. 6 and 7, an insulating support 35 has the base electrode 15 secured thereto so its plane is inclined to the horizontal, e.g., at

30 degrees. Insulating spacer members 36 support the patterned conductive screen electrode 30 parallel to and spaced from the base electrode 15. Similar insulating spacer members 37 serve to hold the substrate 19 and the back electrode 16 (here shown in contact with the substrate 19) in parallel and spaced relation relative to the base electrode 15 and the patterned screen 30. The spacer members 36 are shown as being open at the upper and lower ends of the space between the base electrode 15 and the patterned screen 30 so that developer mixture can be introduced to such space from a hopper 40 for cascading downwardly over the upper surface of the base electrode 15, and the used developer mixture can be removed at the lower end of the base electrode and collected in a basin 41 for mixture with additional toner particles prior to re-use.

In FIGS. 2-5, with the electrodes in horizontal position, the upward flight of the carrier particles 11 and toner particles 12 is against gravity and in the opposite direction to the vector lines representing the force of gravity on the respective particles. In the embodiment illustrated in FIGS. 6 and 7, the flight of the developer mixture 10 is initially at right angles to the base electrode 15 but, because of the inclined position of the electrodes, the particles travel in a path which is the resultant of the electrostatic acceleration vector and the gravitation vector. When a charged carrier particle encounters and is stopped by the screen 30, it will thereafter fall downwardly toward the base electrode 15 and encounter the same at a point relatively downhill from its point of departure.

The practice of the present invention compares favorably with previous methods of coating and printing. The presently disclosed methods offer advantages over conventional printing or coating since, with the present invention, the substrate need not be subjected to pressure and need not be physically contacted by any printing or coating plate or roller. The proposed new methods are also unique in their ability to transfer electrically non-conductive toner particles over relatively greater distances than previous methods without loss and resolution. Distances between the stencil screen and substrate have been increased to one-quarter inch and even one-half inch with but slight loss in resolution. While the substrates have been exemplified as planar sheets of paper, or cardboard, or metal, the invention is not limited thereto and the increased ability to transfer toner particles over longer distances is of particular interest for printing or coating substrates having irregular contours.

One of the greatest advantages of the disclosed invention is the ability to accomplish electrostatic printing through stencil screens with non-conductive toner particles. The use of suchparticles is of particular merit because they contain materials with low melting points and, hence, may be readily fixed by heat, or suitable solvents, to the substrates being coated or printed.

it is apparent from the foregoing that there has been disclosed various unique and noval methods and apparatus for electrostatic coating and printing. Each disclosed embodiment is capable of producing a satisfactory print upon a substrate; however, when a multiple number of prints are being produced, there is a tendency for some of the charged carrier particles and toner particles to adhere to the various screens 17, 25 or 30 such that a charge is built up which exerts a repulsive force against an approaching carrier particle, to slow it down or deflect it. Such an effect is limited by the fact that the various screens are held in place by electrical insulators and thus can dissipate, at a limited rate, the build-up of a charge on the various screens. It is also possible that corona discharges may take place at local areas of the screens. Thus, two mechanisms may exist for limiting the build-up on the screens.

A more expeditious manner of dissipating this undesirable build-up of charge will be apparent from a further consideration of FIGS. 4-7. A voltage divider, generally indicated by the numeral 50, is connected across the power source 20 and switch 21. An adjustable tap 51 on the voltage divider is connected to the conductive portions of conductive screens 25 and 30. Therefore, the conductive screens 25 and 30 are maintained at an opposite polarity relative to the base electrode l5. Carrier particles 11 are originally charged by the base electrode 15 so as to have, for example, a negative charge, and are repelled therefrom so as to contact the screens 25 or 30. Thereafter, the carrier particles in contact with the screen are charged by conduction and repelled by the screen back toward the base electrode 15. Such an arrangement effectively reduces any deleterious build-up charge on the screens 25 or 30.

While preferred forms and arrangement of parts have, been shown in illustrating the invention, and preferred methods have been disclosed for practicing the invention, it is to be clearly understood that various changes in details and arrangement of parts may be made without departing from the spirit and scope of the invention, as defined in the appended claimed subject matter.

I claim:

1. An electrostatic process for applying a selective coating to a substrate comprising the steps of mixing electrically conductive carrier particles with electrically non-conductive toner particles so that the toner particles triboelectrically adhere to the carrier particles, placing said carrier particles and toner particles in a space between said substrate and a base electrode spaced therefrom with said carrier particles being in electrical contact with said base electrode, and establishing an electric field in the space between said substrate and said base electrode for moving said carrier particles and toner particles toward said substrate.

2. An electrostatic process as defined in claim 1 including the step of obstructing the motion of some of said carrier particles and some of said toner particles, and causing the remaining carrier particles and toner particles to continue toward and impinge upon said substrate.

3. An electrostatic process as defined in claim 2 wherein said step of obstructing the motion of some of said carrier particles and some of said toner particles includes the step of interposing an apertured screen in the space between said base electrode and said substrate.

4. Apparatus for coating a substrate with a selective coating of particles, said apparatus comprising a pair of spaced electrodes, a developer mixture comprised of electrically conductive carrier particles and electrically non-conductive toner particles triboelectrically adhered together and located in the space between said electrodes with said carrier particles being in electrical contact with one of said electrodes, an apertured tern, said openings being large enough to allow passage of carrier particles and toner particles therethrough.

8. Apparatus as in claim 4 wherein said substrate is comprised of electrically non-conductive material, said substrate being located between said apcrtured screen and said other of said electrodes.

9. Apparatus as in claim 8 wherein said apertured screen has openings therein for defining a pattern. said openings being large enough to allow passage of carrier particles and toner particles therethrough.

* l l i 

1. An electrostatic process for applying a selective coating to a substrate comprising the steps of mixing electrically conductive carrier particles with electrically non-conductive toner particles so that the toner particles triboelectrically adhere to the carrier particles, placing said carrier particles and toner particles in a space between said substrate and a base electrode spaced therefrom with said carrier particles being in electrical contact with said base electrode, and establishing an electric field in the space between said substrate and said base electrode for moving said carrier particles and toner particles toward said substrate.
 2. An electrostatic process as defined in claim 1 including the step of obstructing the motion of some of said carrier particles and some of said toner particles, and causing the remaining carrier particles and toner particles to continue toward and impinge upon said substrate.
 3. An electrostatic process as defined in claim 2 wherein said step of obstructing the motion of some of said carrier particles and some of said toner particles includes the step of interposing an apertured screen in the space between said base electrode and said substrate.
 4. Apparatus for coating a substrate with a selective coating of particles, said apparatus comprising a pair of spaced electrodes, a developer mixture comprised of electrically conductive carrier particles and electrically non-conductive toner particles triboelectrically adhered together and located in the space between said electrodes with said carrier particles being in electrical contact with one of said electrodes, an apertured screen interposed between said developer mixture and the other of said electrodes, and a power source connected across said electrodes for creating an electric field therebetween for moving said carrier particles and toner particles toward said apertured screen.
 5. Apparatus as in claim 4 wherein said other electrode is a substrate.
 6. Apparatus as in claim 4 wherein said screen is comprised of electrically non-conductive material.
 7. Apparatus as in claim 4 wherein said apertured screen has openings therethrough for defining a pattern, said openings being large enough to allow passage of carrier particles and toner particles therethrough.
 8. Apparatus as in claim 4 wherein said substrate is comprised of electrically non-conductive material, said substrate being located between said apertured screen and said other of said electrodes.
 9. Apparatus as in claim 8 wherein said apertured screen has openings therein for defining a pattern, said openings being large enough to allow passage of carrier particles and toner particles therethrough. 